On 26/05/15 11:59, CK Tan wrote:
> It has to do with the implementation of slot_getattr, which tries to do
> the deform on-demand lazily.
>
> if you do select a,b,c, the execution would do slot_getattr(1) and
> deform a, and then slot_getattr(2) which reparse the tuple to deform b,
> and finally slot_getattr(3), which parse the tuple yet again to deform c.
>
> Where as if you do select c, b, a, it would do slot_getattr(3) to deform
> c, and in the process deform a and b in one pass. Subsequent calls to
> slot_getattr 1 and 2 would find the attribute ready and available, and
> return it (without parsing the tuple again).
>
If this was the case, changing column order would lead to performance
increase, not decrease as reported.
My guess would be same as Amits, it's most likely the additional
projection step.
-- Petr Jelinek http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training &
Services
